Planar illumination apparatuses are often used, for example, for backlights of liquid crystal panels. In general, planar illumination apparatuses of this type are mainly classified into: direct types in which a light source such as a fluorescent light is disposed immediately below a light diffusion plate formed of a plate-like body having a predetermined thickness and area, so that the light diffusion plate is directly irradiated by the light source and the light diffusion plate surface emits light; and edge lighting types in which a light source such as a fluorescent light or LED is disposed on at least one side of a light guide plate formed of a plate-like body having a predetermined thickness and area to allow the light guide plate surface to emit light.
Of these planar illumination apparatuses, the direct-type illumination apparatus is structured to have a prescribed gap, that is, a predetermined distance between the light source and the light diffusion plate. However, if this distance is reduced, the light diffusion plate reflects the outer shape of the light source, which makes visibility poor, resulting in degradation of illumination quality. If a point light source having strong directivity is used as the light source, the luminance becomes extremely high at the light diffusion plate immediately above the point light source, thereby causing a luminance difference with other illumination areas, and as a result, uniform illumination light cannot be obtained. One of the possible methods for solving these problems is to increase the distance between the light diffusion plate and the light source. However, this method causes new problems. For example, it is difficult to obtain illumination light having a desired illuminance because the whole area becomes darker as the distance increases, and it is impossible to reduce the thickness. Because of the problems above, it is difficult to adopt direct-type illumination apparatuses in some applications.
Therefore, the edge lighting illumination apparatuses have been used in place of such direct-type illumination apparatuses and proposed in many cases.
For example, Patent Document 1 below discloses an edge lighting illumination apparatus using a light emitting diode (hereinafter referred to as “LED”) as a light source. This illumination apparatus includes an LED, a light guide plate having a light introduction portion formed on a flat surface, and a reflecting mirror that reflects light from the LED. The LED is mounted on the flat surface of the light guide plate and is covered with the reflecting mirror. The irradiation light from the LED is reflected by the reflecting mirror and introduced to the light guide plate. With this illumination apparatus, the irradiation light from the LED is efficiently taken into the light guide plate. Patent Document 2 below discloses an illumination apparatus configured to include a light source device having an LED and a light source rod, and a light guide plate for guiding irradiation light from the light source device. The light source rod is formed of a prism array having a predetermined shape. The light source rod allows the irradiation light from the LED to be emitted to a target through the light guide plate, thereby making luminance uniform. Patent Document 3 below discloses a cash register guide lamp in which a plurality of LEDs are arranged at regular intervals on a light entrance surface of a light guide body, light from these LEDs are irregularly reflected at a reflector, and the scattered light causes a light exit surface of the light guide body to emit planar light to illuminate a display body opposed to the light exit surface of the light guide body.
Patent Document 4 below discloses an edge lighting planar light source that emits planar illumination light, for example, using a linear light source. With reference to FIG. 14, the planar light source disclosed in Patent Document 4 will be described below. FIG. 14A is an exploded perspective view of a box that comprises the planar light source described in Patent Document 4, and FIG. 14B is a sectional view of FIG. 14A cut in the longitudinal direction. This planar light source includes a box 20 having an inner surface formed as a light reflecting surface, and a light source 10 contained in the box. A plurality of light transmission regions 30 are evenly provided on a top wall surface 20a that is one of the wall surfaces of the box. The proportion of these light transmission regions 30 to one wall surface increases as the distance from the light source increases. The light source light is emitted from the top wall surface 20a. 
An example of this planar light source is described as follows. The box is formed of a metal plate coated with light reflecting metal film such as aluminum with a thickness of about 0.2 to 1.0 mm, and a light emission surface thereof approximately has length A×width B of 100 mm×100 mm and a depth of 5 mm. The light transmission regions 30 of the top wall surface include through holes having a diameter of about 0.4 mm at minimum to about 0.8 mm at maximum that are evenly provided at 1-mm pitches. A fluorescent light having a diameter of 3 mm and a length of 100 mm with luminance of 28,000 cd is used as the light source 10. This fluorescent light is arranged at one end portion of the box 20, and the top wall surface 20a seals the main body to prevent leakage of light. A light diffusion plate (not shown) made of polyethylene terephthalate (PET) or polycarbonate is provided on the top wall surface 20a. The light source includes a point light source such as a bulb, a halogen lamp, or a light emitting diode, a linear light source formed by arranging rod-like light sources such as fluorescent lights or point light sources for emitting light over an elongated extent, and a ring-shaped light source.
It is described that this planar light source achieves the effects as follows. In this planar light source, a light source is contained inside a box having an inner surface formed as a light reflecting surface, and light exits from the light transmission regions while being reflected by the inner surface in the interior space of the box. Therefore, light from the light source can be taken into the box completely without any loss. In addition, light absorption in the space is very small, and light from the light source is taken out from the light transmission regions almost completely and contributes as the planar light source without loss produced when light is taken into a conventional light guide plate or light absorption loss due to passage through the light guide plate. Thus, the light use efficiency is greatly improved. The proportion of the light transmission regions is small with respect to the wall surface from which light is taken out in the proximity to the light source, whereas the proportion becomes larger as the distance from the light source increases. Therefore, at a location close to the light source and with a large quantity of light, light is emitted from a part where the proportion of the light transmission regions is small, whereas at a location distant from the light source and with a small quantity of light, light is emitted from a part where the proportion of light transmission regions is large. As a result, light with uniform luminance is emitted from the surface.